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We often find that people have a number of technical questions about fuses for circuit protection. Whether you need to know the basics, such as when to use a fuse, or to consider more complex issues such as I2t or Watts loss, we have put together these FAQs to answer some of your queries.
Remember that our technical team are always ready to answer whatever questions you may have.
This number is the largest overcurrent that a device can deal with safely. This means that if a fuse is rated at 100kA then it can deal with any overcurrents at or below this number safely. A fuse should not be used in a circuit where the known short circuit current exceeds the maximum braking capacity of the fuse.
This is an extremely important figure in fuse specification. I2 is current squared whereas t is time. The figure shows how quickly the fuse reacts to different amounts of energy passing through it.
Or to put it another way, the I2t figure shows the amount of energy needed to operate the fuse. When you are specifying a fuse, you need to make sure that its I2t figure is smaller than the I2t of the equipment that it is protecting.
Why is it important to consider the watts loss of a fuse?
Watts loss is the energy lost when a current passes through the fuse due to the heating effect during normal running of the fuse link. It shows how hot a fuse gets in normal operation. In order to save energy, we need to keep the watts loss figure of the fuse as low as possible. It is also important because we don’t want too much heat building up in the panel under normal operating conditions.
To explain this let’s briefly return to how a fuse works; for it to blow the metal element, typically copper, needs to melt. If the fuse needs to interrupt a lower overload fault current or short circuit, then we might need to lower this melting temperature as the heating effect at lower currents is smaller. Instead of constructing a very thin copper link, we can add a little bit of solder alloy to reduce the melting temperature.
We call this the M-Effect. The exact M-Effect will vary according to how the fuse operates and what size of overload or fault current it needs to break.
What effect does the ambient temperature have on a fuse?
Providing that the ambient temperature is between -20oC and 35oC there is no effect. If the temperature gets above 35oC then this could affect the build-up temperature in the element of the fuse and therefore it’s operation. It means that you may need to apply a derating factor on the fuse you specify so that it will still operate in these conditions. Ongoing high ambient temperatures may also affect the lifespan of the fuse.
If you need a fuse to operate in either high or very low ambient temperatures, then it could be worthwhile speaking to the fuse manufacturer.
What standards are fuses made to?
The most common standard that you should check for is IEC 60269. This is the international standard from which most other standards such as EN or BS are based upon.
There are separate standards for UL, covering North America, but we would suggest that you speak to your fuse manufacturer if you need to export to this area because products can be UL listed, which means that samples of it have been tested and meet applicable standards, or UL Recognised, which applies to components that need to be incorporated in specific ways to be safely used.
An Arc flash is an extremely large current that can pass through the air between conductors, or from a conductor to a ground source. This means that if a cable melts or vaporises then this current could still pass through the air.
Fuses are designed to prevent this from happening which is why they are often used as protection against very high overload or fault currents and protect against arc flash. Arc flash is only a risk when working on live circuits. The fuse and protective devices should be rated to reduce this, but also the correct PPE should be worn in such cases.
What are the differences between gFF, gG, gR, aM and aR fuses?
These letters all refer to the IEC international standard. They are all different types of fuses and relate to how they work.
What is the difference between 415 V a.c. and 690 V a.c. fuses?
415V volts tends to be the industry standard voltage in the UK, but there are some bigger installations that need a higher voltage, for example if there are very long cable runs.
It’s important to know what the voltage rating is for an installation because you should never use a fuse beyond its stated rating. So, you can use a 690V fuse for lower voltage applications such as 415V, but you must not use a 415V fuse for higher voltages. 690V fuses tend to be larger and more expensive than the 415V options.
What is the difference between DC and AC?
This describes the type of voltage and current flow in a circuit. In DC, or Direct Current, the current only flows in one direction and is “always on”. For example a dc circuit at 50V will remain at 50V continuously until switched off. With AC, or Alternating Current, on the other hand it changes direction many times per second, typically 50 times a second. This means that for alternating currents the voltage also reverses when it changes direction.
This is important because alternating current will pass though zero volts at some point. Ac is used for most applications and fuses generally have an a.c voltage rating. As the voltage and current pass through ero (typically 50 times a second) it is slightly easier for a fuse to break this voltage and current in a faulted circuit. When you try break the circuit of a DC current it will carry on flowing in the same direction as there is no zero point. It will try to continue to flow and is very difficult to break. D.c rated fuses are generally longer as they are designed to handle difficult d.c voltages and currents.
What does a time current curve show you?
This is a graph that plots time on the y axis against current on the x axis. The graph shows you the operating characteristics of the fuse - or put another way how quickly the fuse will operate at a given fault current. So, higher overload currents will result in the fuse links element melting or vaporising more quickly.
If you look at the graph then any operating conditions to the left of the curve leaves the fuse intact, but anything to the right means the fuse has operated.
